The efficiency of a LED is limited by the total internal reflection because the index of refraction of the semiconductor material making up the LED (typically 3-3.5) is much higher than the index of refraction of the air (1) into which the light is emitted. Light distribution within a LED is generally referred to as Lambertian distribution, or omnidirectional distribution. Because of this large spatial distribution of the generated light, only small amounts of the light can be effectively utilized, especially if the light is going to be coupled to a light conduction medium such as an optical fiber. In dealing with a Lambertian source, such as a LED, there are many ways available today to provide a method to increase the light coupling from the LED to the fiber core. For example, spherical lenses, microlenses, gradient index lenses (GRIN), diffraction gratings all are utilized successfully. Unfortunately, none of these techniques actually increases the amount of light generated for the same input current.
Increased efficiency implies less current and, therefore, less power to couple the same amount of light to a system. A more efficient LED provides higher optical coupling and less stringent specifications on the coupling designs and procedures, which results in less expensive and more manufacturable systems. These reasons imply that if the efficiency of a LED can be increased then the device will become more useful in applied technology.
One method to improve the external efficiency of an LED is to utilize mirror stacks on opposite sides of an active area to provide multiple reflections of the photons inside the LED cavity. This technique has been successfully applied to vertical cavity surface emitting lasers (VCSEL) where two mirror stacks are reflective enough to generate stimulated emission. The design of such mirror stacks can be engineered to produce a superluminescent LED, as disclosed in a co-pending U.S. patent application entitled "Superluminescent Surface Light Emitting Device", filed Oct. 4, 1991, bearing Ser. No. 07/770,841 and assigned to the same assignee.
A major difference between superluminescent LEDs and standard LEDs is the fact that the spatial distribution of light output is much narrower and allows for higher efficiency optical coupling to fibers, waveguides, etc. In VCSELs the spatial distribution of light is extremely narrow. Currently, LEDs require large drive currents to generate enough light so that a reasonable amount of light couples to an optical fiber. With improved efficiencies, less current is required and, therefore, less heat is generated. Thus, a LED with improved efficiency results in greater applicability of the LED for commercial use.